Dr. Doom tries to trick Spider-Man into helping him defeat the Fantastic Four. When Spider-Man turns him down, he decides to capture Spider-Man. Flash Thompson dresses up as Spider-Man for a prank and Dr. Doom mistakes him for the real Spider-Man and captures him instead, and Flash has to be saved by the real Spider-Man.
This story is an example of Doctor Doom using a robot to take his place. The first instance of this was seen in Fantastic Four (Marvel, 1961 series) #5.